In communication applications such as high frequency transmission and reception, including radio frequency (RF) transmission, several electronic components are used. For example, in a classical transmission system or chain, digital modulated inputs are converted into the analog domain to be transmitted through the atmosphere. FIG. 1 shows one example of a transmission chain 10, in which a digital signal 12 is received by a digital modulator 14, which converts the information in the digital signal into a digital continuous-time waveform for transmission. A digital-to-analog converter (DAC) 16 receives the output of the digital modulator 14, and converts the digital waveform into an equivalent analog waveform. A filter 18 receives the analog waveform and filters spurious frequencies that are introduced in the conversion process used by the DAC, since this process is not a perfect one. For example, a transconductance filter is commonly used for filter 18. The filtered analog waveform is then received by an RF block 20, which includes a mixer for ensuring a conversion into the RF domain and a power amplifier for driving a connected antenna 22, and is able to transmit the analog waveform into the atmosphere using the connected antenna 22 or similar mechanism. This type of transmission chain can be used with various different standards, including IEEE 802.11, Bluetooth, Global System for Mobile Communications (GSM), and other standards.
The interfacing between the DAC 16 and the filter 18 is a critical issue in the transmission chain 10, since a poor compatibility between these components can induce distortions in the transmitted signal and cause the signal to disrespect the output spectral masks imposed by a particular standard.
One common configuration for these components in the transmission chain 10 is to use a current-steering DAC for DAC 16, and a transconductance filter for filter 18, specifically a transconductance-C (Gm-C) filter built with Nauta's transconductance. Such a configuration allows several advantages, including low cost, low-power operation and linearity. Nauta's transconductance is often used in a transmission chain as well as reception chains, and offers advantages of low supply voltage, internal common mode regulation, and no internal nodes, such that no compensation capacitor is required.
An example of a typical current-steering DAC 50 is shown in FIG. 2, shown in a single-ended configuration. The DAC 50 is generally used in a differential configuration, and thus a second DAC component would be provided for the differential layout. DAC 50 includes a digital input 52 from the digital modulator 14 that provides a digital signal to the DAC 50. Input 52 is connected to unit current sources 54, where a number of the current sources 54 are turned on based on the received digital code at input 52, and where N is the number of bits at the digital input 52. The currents from the current sources 54 are summed to provide an output current proportional to the digital input 52, as is well known in DAC operation. The analog output voltage from current sources 54 is generated at branches of a resistive load RL which is connected to a reference voltage of the DAC called VREF_IDAC. The analog output voltage VIP can be expressed as shown in equation (1):VIP=VREF_IDAC−n·Iu·RL  (1)where Iu is the unit current source and n is the number of unit current sources turned on. The number n can generally be any number between zero and (2N−1).
FIG. 3 is a schematic illustration of an example of a typical transconductor 70 of the transconductance filter 18. Transconductor 70 is a Nauta's type of transconductor, and includes two differential input terminals INP and INN. Each input is connected to an inverter 72, where each inverter 72 has a supply control voltage VCTRL. Additional inverters 74 are coupled between the input terminals as shown, and also receive the supply control voltage VCTRL. The transistors of the inverters 72 and 74 are CMOS n and p transistors. Two output terminals OUTN and OUTP are provided as the differential output of the transconductor 70.
The transconductance value (gmd) of transconductor 70 depends on the supply voltage VCTRL and on process parameters, as shown in equation (2):gmd=(VCTRL−Vtn+Vtp)√{square root over (βnβp)}  (2)
where Vtn and Vtp are the threshold voltages of the n and p CMOS transistors of the transconductor 70, VCTRL is the voltage supply of the transconductor and the Gm-C filter 18, and
      β          n      ⁢              /            ⁢      p        =            μ              n        ⁢                  /                ⁢        p              ⁢    Cox    ⁢                  W        L            .      
FIG. 4 is a block diagram illustrating the Gm-C filter 18. Since the Nauta's transconductance value of the transconductor 70 depends on the supply voltage VCTRL, VCTRL is the tuning voltage of the filter 18. A frequency automatic tuning block 80 automatically tunes the VCTRL supply for the filter 18. For example, a standard phase locked loop can control the frequency automatic tuning, so that the VCTRL supply moves in such a way that the cutoff frequency of the filter 18 is provided at a desired frequency rather than a frequency influenced by the process variations of the circuits (i.e., so that the cutoff frequency is process independent). The transconductor 70 is shown as a schematic symbol provided within the filter 18. The filter 18 receives VIP and VIN input signals, and provides IPFILT and INFILT filtered output signals.
FIG. 5 is a block diagram illustrating a circuit 90 including the DAC 92, transconductance filter 18, and the connection of the DAC 92 to the filter 18. The two differential components 50a and 50b of the DAC 92 are connected to the VIP and VIN inputs of the filter 18, respectively. The outputs IPFILT and INFILT of the filter 18 are provided to the RF block 20 as shown in FIG. 1.
One problem with the prior transmission chains that use a current-steering DAC and a transconductance-C filter is that the interfacing between the DAC and the filter is not optimal. The common mode compatibility between these components is typically poor, which induces distortions and noise in the transmitted signal and causes the signal to disrespect the output spectral masks imposed by a particular communication standard. This can also lead to sub-optimal linearity and power consumption in the transmission system performance.
Accordingly, improved common mode management between a current-steering DAC and a transconductance filter in an RF transmitter would reduce distortion in transmitted signals.